The application of urethane technology to produce polyurethane shoe soles attached to uppers coated with nitrocellulose-containing finish is widely known. Shoe producers are very interested in producing this type of shoe because of the desirable feel and style associated with nitrocellulose coated uppers. However, when a nitrocellulose finished upper is attached to a polyurethane sole, a yellowing or discoloration of the nitrocellulose finished upper occurs. This yellowing, or staining, is most noticeable when white shoes are produced but can be seen with all colors of nitrocellulose coated uppers.
Tertiary amines are customarily used as catalysts in polyurethane shoe sole compositions. Normally the catalyst is blended with the polyol, surfactant and water, which blend in turn is reacted with the polyisocyanate to produce polyurethane shoe soles. Triethylenediamine is used widely and most predominantly by the shoe sole industry. The molecular structure and size of the triethylenediamine molecule permits the catalyst to be highly mobile and "move" throughout the polymer ensuring complete catalysis. The added green strength allows the shoe sole molders to demold the soles quickly.
It has been discovered that the heat produced in the curing of the sole causes the amine to volatilize and migrate to the surface, attacking the nitrocellulose coated part causing the nitrocellulose staining.
One attempt at solving the staining problem focused on using catalysts that contain a reactive hydroxyl functionality. The reactive hydroxyl group would react into the polymer network and not permit migration to the surface, thus eliminating the attack of the nitrocellulose. However, the decreased reactviity of this type of catalyst resulted in extended demold times to a point where it became economically unfeasible because of increasd production cost. Demold times were extended from 3 minutes to about 7 or 8 minutes when using such relative catalysts. In addition, the physical properties of the polyurethane shoe soles were also negatively affected.
At present, manufacturers are warned against the use of nitrocellulose-containing finishes on shoe uppers with polyurthane shoe soles as the only acceptable method to avoid a staining problem.
U.S. Pat. No. 4,486,370 discloses the addition of a small amount of a halogen compound, e.g. chloroethyl phosphate, in a process for producing elastomeric poluyrethane RIM parts using tertiary amines as the chief catalyst component results in scavenging the residual amines after the part has been cured by annealing. The parts from such a process can be coated with a wide variety of enamels and completely cured to result in a higher degree of adhesion between the coating and the surface of the part compared to those parts from a process not using such a halogen compound.